1. Field of the Invention
The present invention relates to a method, system and network device for routing a message, e.g. a Multimedia Messaging Service (MMS) notification, to a temporarily unavailable network user, e.g. a subscriber in an Internet Protocol Multimedia Subsystem (IMS).
2. Description of the Related Art
In order to achieve access independence and to maintain a smooth interoperation with wired terminals across the Internet, the IMS as specified e.g. in the 3GPP specification TS 23.228 has been developed to be conformant to IETF (Internet Engineering Task Force) “Internet Standards”. The IP multimedia core network (IM CN) subsystem enables network operators of mobile or cellular networks to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols. The intention is to develop such services by mobile network operators and other 3rd party suppliers including those in the Internet space using the mechanisms provided by the Internet and the IM CN subsystem. The IMS thus enables conversions of, and access to, voice, video, messaging, data and web-based technologies for wireless users, and combines the growth of the Internet with the growth in mobile communications.
FIG. 1 shows an architecture of an IMS network according to the above 3GPP specification. The architecture is based on the principle that the service control for home subscribed services for a roaming subscriber is in the home network HN, for example, a Serving Call State Control Function (S-CSCF) is located in the home network HN. In FIG. 1, a current or old S-CSCFo 10 and a future or new S-CSCFn 12 are shown, between which a terminal device or user equipment (UE) 40 can be transferred e.g. due to changed required capabilities resulting from a change in the subscriber profile or network coverage of the UE 40.
In general, an S-CSCF performs the session control service for the served UEs. It maintains a session state as needed by the network operator for support of the services which may be provided by an application server (AS) 60 which may be located as well in the home network HN or a visited network VN. Within an operator's network, different S-CSCFs may have different functionalities. The functions performed by the S-CSCF during a respective session are, for example, registration, session flow management, charging and resource utilization management. When a subscriber roams to the visited network VN, the visited network VN supports a Proxy-CSCF (P-CSCF) 30 which enables the session control to be passed to the respective S-CSCF located at the home network RN and provides the service control. Furthermore, an Interrogating-CSCF (I-CSCF) 50 is provided in the home network HN as a contact point within the operator's network for all connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator's service area. There may be multiple I-CSCFs within an operator's network. The functions performed by the I-CSCF 50 include assigning an S-CSCF to a user performing a registration procedure, routing a request received from another network towards the S-CSCF, maintaining the address of an S-CSCF from a subscriber database, e.g. a Home Subscriber Server (HSS) 20 as shown in FIG. 1, and/or forwarding requests or responses to the S-CSCF determined based on the address of change from the HSS 20.
The P-CSCF 30 is the first contact point within the IMS. Its address is discovered by the UE 40 following a PDP (Packet Data Protocol) context activation. The P-CSCF 30 behaves like a proxy. Namely, it accepts requests and services them internally or forwards them on, possibly after translation. The P-CSCF 30 may also behave as a User Agent. In other words, in abnormal conditions, the P-CSCF 30 may terminate and independently generate transactions. The P-CSCF 30 forwards register requests received from the UE 40 to an I-CSCF, for example, the I-CSCF 50, determined using the home domain name as provided by the UE 40. The P-CSCF 30 forwards requests or responses to the UE 40.
Further details regarding the functions of the different CSCF elements shown in FIG. 1 can be gathered from the above mentioned 3GPP-specification.
The IETF has been specifying a Session Initiation Protocol (SIP) event package for registrations, as defined in “draft-ietf-sipping-reg-event”. Through its REGISTER method, the SIP allows a user agent, which is an interface (e.g. browser) between the user and the network application, to create, modify, and delete registrations. Registrations can also be altered by administrators in order to enforce policy. As a result, these registrations represent a piece of state in the network that can change dynamically. There are many cases where a user agent would like to be notified of changes in this state. The event package defines a mechanism by which those user agents can request and obtain such notifications.
The SIP REGISTER method provides a way for a user agent to manipulate registrations. Contacts can be added or removed, and the current set of contacts can be queried. Registrations can also change as a result of administrator policy. For example, if a user is suspected of fraud, his registration can be deleted so that they cannot receive any requests. Registrations also expire after some time if not refreshed. Thus, registrations represent a dynamic piece of state maintained by the network. The SIP Events Framework defines a generic framework for subscription to, and notification of, events related to SIP systems. The framework defines the methods SUBSCRIBE and NOTIFY, and introduces the notion of a package. A package is a concrete application of the event framework to a particular class of events, e.g. registration states.
The SUBSCRIBE message for the registration package may contain a body for filtering the subscription. It may be sent with or without the body. The default registration policy is that notifications are triggered from a SUBSCRIBE message and are generated every time there is a change in the state of any of the registered contacts for the resource being subscribed to. Those notifications only contain information on the contacts whose state has changed. The notifications are forwarded using the NOTIFY message, which includes in its body a registration information document that describes some or all of the contacts associated with a particular address-of-record.
In the 3GPP IMS Release 5 specifications TS 24.229, 24.228 and 23.218, the SIP registration state event package is used to inform the subscribers of the event package about the user's registration state. The 3GPP IMS Release 6 may introduce new services to the system, such as Presence, Messages, Conferencing and MMS. There are services like MMS which can utilize the capabilities of the IMS network. The IMS network is able to provide accurate information on the user's registration state using the SIP registration state event package and it also carries the MMS notification using e.g. the SIP MESSAGE request.
According to the IMS Release 5 specifications, the IMS subscriber is either registered or deregistered. However, although being registered, the IMS subscriber may not be reachable e.g. due to battery loss, temporary radio coverage loss at the current location of the subscriber, which is quite normal in big cities or areas where the radio coverage is for some reasons spotty. Consequently, external services utilizing the IMS network cannot be notified if a service user in not reachable.